1. Field of the Invention
The invention relates to a CRT (cathode ray tube) with a colour selection electrode having rows of elongated openings. The colour selection electrode is also called a xe2x80x98maskxe2x80x99. More in particular it relates to a CRT with a tensioned colour selection electrode in which tension is applied in a direction and the colour selection electrode comprises openings elongated in said direction, the openings being separated from each other in said direction by bridges.
2. Description of the Prior Art
In U.S. Pat. No. 4,942,332 a slit-type flat foil tension mask is described having slits with large (in comparison to conventional masks) longitudinal dimensions. Typically the length of the slits is of the order of 1 inch (2.5 cm). The distance between bridges between slits is sometimes also called the vertical pitch or in short av.
The bridges in between the slits provide mechanical strength to the tensioned colour selection electrode, without substantially reducing the image brightness or deforming the mask when the mask is put under tension. However, they also pose a problem in that the bridges may be visible as two straight horizontal lines, reducing the image quality.
In U.S. Pat. No. 4,942,332 a solution for this problem is described, being constituted by randomising the length (pitch) of the slits.
It is proposed in xe2x80x98Stretched tension mask with large vertical pitch for CMTsxe2x80x99 IDW 99, page 573-576 to use, in order to remove or at least reduce, the visibility of the bridges a vertical pitch from 2.4 mm to 6 mm for a 21xe2x80x3 CRT. Smaller vertical pitches (between 1 and 2.4 mm) provide large Moire patterns and are thus excluded.
Both of these solutions, however, have shortcomings, in both the visibility of the bridges is still appreciable.
The inventors have realised that randomisation of the position of the bridges as described in U.S. Pat. No. 4,942,332 leads to unexpected problems. At some parts of the image the bridges become visible, namely there where a number of bridges happen to be at the same horizontal position, or where adjacent bridges happen to be spaced at such distances that Moirxc3xa9 effects occur, where at other parts they are not. The randomisation in fact does not change the distance between the bridges seen in a horizontal dimension by a large amount. The randomisation as described in U.S. Pat. No. 4,942,332 is for example 0.02xe2x80x2, which means that on average the distance between adjacent bridges is 0.01xe2x80x2 or 0.25 mm, and because of the randomisation the distance is sometimes much less. This still leads to lines being visible in the image, to which lines the human eye is very sensitive. Randomising invariantly also leads to clustering, resulting being visible in parts of the image. The human eye is very sensitive to such irregularities in the image. The image is perceived as xe2x80x98patchyxe2x80x99 because of this effect. In some sense this problem is a greater problem than a straight line being visible. The straight line is always there and it is a problem that the viewer understands and will most likely be at least to some degree be visible when the device is bought and will effect all modes of image reproduction in more or less the same degree. The xe2x80x98patchyxe2x80x99 image due to randomisation is something that is dependent of the image that is displayed, and also on the particular mode (VGA, UGA, XVGA etc) with which the image is displayed. These problems become usually visible at higher solution of the image, i.e. the higher the image quality. Such problems manifest themselves more often than not after sale and effect in particular the high quality image modes, leading to the clearly unwanted effect that the xe2x80x98lower image qualityxe2x80x99 in fact gives a higher quality image than the xe2x80x98high quality imagexe2x80x99.
A vertical pitch between 2.4 and 6 mm (as in the above cited article) does reduce the visibility of the bridges, but they are nevertheless visible, especially in a CMT (Colour Monitor Tube, that is a CRT for a computer monitor).
The present invention has as an object to provide a CRT in which some or most of the above cited problems are resolved or reduced, more in particular to provide a CRT in which the presence of the bridges is less noticeable.
To this end the CRT in accordance with the invention is characterised in that the number of slits per row lies between 160 and 180 slits per row for a diagonal dimension of the CRT equal to or larger than 21xe2x80x3 or between 210 and 240 for a diagonal dimension smaller than 23xe2x80x3.
The inventors have realised that, although the resulting vertical pitch or pitches is for a 21xe2x80x3 CRT well within the range for which in the cited article Moire patterns are predicted, the indicated range surprisingly enough provide for a CRT which, when used for either TVT or CMT application shows little or no Moire patterns, while yet offering increased intensity of the image, and strong masks, but no visible effects of individual bridges.
The first indicated embodiment (the number of slits per row lying between 160 and 180 slits per row for a diagonal dimension of the CRT equal to or larger than 21xe2x80x3) provides a CRT which shows little or no Moire for an NTCS or PAL or VGA signal and only small Moire for an SVGA signal as will be explained below. NTSC, PAL and to a lesser degree VGA signal are typically signals for TVT applications. The size of the CRT (larger than 21xe2x80x3) makes for a comfortable viewing distance (roughly three to seven times the height of the image) at which viewing distance the resulting distance between the bridges is such that individual bridges are no longer or less visible.
The second embodiment provides for a CRT which shows little or no Moire for an SVGA or XGA signal which are signals typical for CMT applications. For such applications the viewing distances are typically smaller, but because the number of slits (as compared to the first embodiment) is also increased, visibility of individual bridges is still small.
The invention is amongst others based on the insight that, although in general Moire patterns do indeed cause severe Moire patterns in the range (1 to 2.4 mm) indicated in the cited article, the ranges of the embodiments of the present inventions do not show appreciable Moire patterns. Namely, in the range from 1 to 2.4 mm there is a substantial variation in the intensity and wavelengths of the Moire patterns. In the range of the embodiments of the present invention the Moire intensities are lowest and the Moire is least visible.
Preferably the height of the bridges is for the first embodiment less than 100 micrometer, for the second embodiment less than 70 micrometer.
The inventors have released that, for tension masks with relatively large slits as in the present invention, the height of the bridges may be considerably less than the height of the bridges in conventional shadow masks, which lie around 140 micrometer. This reduction in height reduces the negative influences of the bridges on the image intensity and Moire patterns, thus increasing the image quality.
The invention furthermore relates to a cathode ray tube for TVT applications with a tensioned colour selection electrode in which electrode tension is applied in a direction and the colour selection electrode comprises elongated openings in said direction, the openings being separated from each other in said direction by bridges, characterised in that the number of slits per row lies between 160 and 180 slits per row and to a cathode ray tube for CMT applications with a tensioned colour selection electrode in which electrode tension is applied in a direction and the colour selection electrode comprises elongated openings in said direction, the openings being separated from each other in said direction by bridges, characterised in that the number of slits per row lies between 210 and 240 slits per row.
It is remarked that xe2x80x98for TVT applicationsxe2x80x99 and xe2x80x98for CMT applicationsxe2x80x99 in the art of CRT""s is more than an indication of possible use. CRT""s are specifically made for such applications, in fact the international code for CRT""s indicates (by means of an M) which CRT""s are for CMT applications. Furthermore CRT""s for CMT applications have to comply with certain radiation requirements (due to the relatively small viewing distance) and safety requirements whereas CRT""s for TVT applications have less stringent requirements. Handbooks and brochures also specifically indicate which CRT""s are for which application.
The invention also relates to a TVT comprising a cathode ray tube with a tensioned colour selection electrode in which electrode tension is applied in a direction and the colour selection electrode comprises elongated openings in said direction, the openings being separated from each other in said direction by bridges, characterised in that the number of slits per row lies between 160 and 180 slits per row.
The invention also relates to a CMT comprising a cathode ray tube with a tensioned colour selection electrode in which electrode tension is applied in a direction and the colour selection electrode comprises elongated openings in said direction, the openings being separated from each other in said direction by bridges, characterised in that the number of slits per row lies between 210 and 240 slits per row.
xe2x80x98Between x and yxe2x80x99 includes within the framework of the invention a number of y (or y) slits. Within the framework of the invention some of the outermost rows may have a smaller number of slits.